All photosynthetic organisms depend on the light-harvesting reactions of photosynthesis for energy to produce important compounds for growth and metabolism. Energy-rich carbohydrates, fatty acids, sugars, essential amino acids, and other compounds synthesized by photosynthetic organisms are the basis of the food chain on which all animal life depends for existence. Photosynthetic organisms are also the major source of oxygen evolution in the atmosphere, recycling carbon dioxide in the process. Thus life on earth is reliant on the productivity of photosynthetic organisms, especially plants.
Plant productivity is limited by the amount of resources available and the ability of plants to harness these resources. The conversion of light to chemical energy requires a complex system which combines the light harvesting apparatus of pigments and proteins. The value of light energy to the plant can only be realized when it is efficiently converted into chemical energy by photosynthesis and fed into various biochemical processes.
The thylakoid protein apparatus responsible for the photosynthetic conversion of light to chemical energy is one of the most complex mechanisms in the chloroplast and remains one of the most difficult biological systems to study. Oxygen-evolving photosynthetic organisms, such as cyanobacteria, algae and plants, possess two photosystems, PSI and PSII, which cooperate in series to acquire electrons from H.sub.2 O and deliver them energetically up a gradient to NADP.sup.+. The photosynthetic production of NADPH and ATP then, in turn, feeds into all biochemical pathways. The force driving the uphill flow of these electrons comes from the light energy absorbed by the 100-300 chlorophyll molecules associated with the two photosystems. An important pair of chlorophyll a molecules in the center of each photosystem modulates the movement of electrons. The remaining chlorophyll molecules are associated with proteins which in turn are organized into light gathering antennae that surround the reaction centers and transfer the light energy to them (Green et al. (1991) TIBS 16:181).
The capacity to absorb light, especially in shade, depends largely on the size and organization of the light harvesting complexes (Lhc) in the thylakoid membranes. The LhcII light harvesting complex is the major ensemble of chlorophyll a/b binding protein (Cab) acting as an antenna to photosystem II (PSII) and plays a key role in harvesting light for photosynthesis (Kuhlbrandt, W. (1984) Nature 307:478). Plants are capable of adjusting the size of the antennae in accordance with the light intensity available for growth. In shade, the allocation of nitrogen is shifted from polypeptides in the stroma, by decreasing ribulose 1,5-bisphosphate carboxylase (Rbc or Rubisco) levels, to the thylakoidal proteins. Nitrogen redistribution is a compensating response to low irradiance, balancing light harvesting and CO.sub.2 fixation (Evans, J. R. (1989) Oecologia 78:9); Stitt, M. (1991) Plant, Cell and Environment 14:741).
In addition to the shift in the investment of nitrogen into different proteins, photosynthetic organisms can adapt to low light conditions by molecular reorganization of the light harvesting complexes (Chow et al. (1990) Proc. Natl. Acad. Sci. USA 87:7502; Horton et al. (1994) Plant Physiol. 106:415; Melis, (1991) Biochim. Biophys. Acta. 1058:87). A plant's reorganizational ability to compensate for changes in the characteristics of the light limits its productivity. Although a mechanism is in place to adapt to low light conditions, photosynthesis in plants grown in suboptimal illumination remains significantly lower due to a limited capacity to generate ATP and NADPH via electron transport (Dietz, K. J. and Heber, U. (1984) Biochim. Biophys. Acta. 767:432; ibid (1986) 848:392). Under such conditions the capacity to generate ATP and NADPH, the assimilatory force, will dictate the capacity to reduce CO.sub.2. When light is limiting, plants reorganize to maximize their photosynthetic capacity; however, the ability to adapt is limited by molecular parameters ranging from gene expression to complex assembly to substrate and cofactor availability.
If productivity of a plant or other photosynthetic organism is to be increased, methods to enhance the light-gathering capacity without restricting CO.sub.2 fixation must be developed.